[Mp4-tech] encoder and decoder output different motion vectors

[Tomer Segal]

hi,
i'm a little confused about a set of results i got using the mpeg4 reference software (MoMuSys-1.0-001220_sony).
i encoded a yuv sequence having the encoder output the motion vectors calculated during encoding. then decoded the resulting mpeg file with the decoder, enabling a trace of the motion vectors extracted during  decoding. i repeated these steps twice, once for a very simple yuv sequence (few blocks change throughout the sequence) and a second time using a more complex sequence. for the simple sequence the motion vectors output by the encoder and the decoder were exactly the same. for the complex sequence however there were differences between the vectors - in most blocks vectors differed by a single halfpel, but a few of them differed by as much as 5 halfpels.
i thought the motion vectors extracted by the decoder were suppose to be exactly the same as the ones calculated in the encoder. am i wrong? i tried finding the answer in the mpeg4 iso and other related articles but i was unable to.
if indeed the vectors are supposed to be identical, what could be causing this difference? 
Thanks,
    Tomer.
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